Piezoelectric fans operate as a vortex shedding device. U.S. Pat. No. 4,498,851 nicely describes vortex shedding as a process where air is prevented from being sucked around a piezoelectric fan blade tip when its motion reverses. Vortex shedding is based on the fact that air displaced from the front of a moving blade rotates so rapidly that the air is unable to reverse its direction of rotation when the blade reverses its motion. If the rotation is not sufficiently rapid, the vortex can reverse its direction of rotation to be sucked around the blade tip instead of leaving the blade.
The vortex shedding action is illustrated in FIGS. 1A-1I. In FIG. 1A, a blade 10 of a piezoelectric fan is centered and moving upward at maximum velocity as indicated by arrow 12, and air is being sucked downward around the blade tip as indicated by arrow 14. While this is happening, a previously shed vortex 16 is moving to the right below a center line 18 of the blade (the center line being when the blade 10 is at rest). In FIG. 1B, the blade 10 is beginning to curve upward at about one quarter amplitude. The air is being sucked around the blade tip into a vacuum on the back (lower per the orientation in FIG. 1B) side of blade 10 and the new vortex 14a is beginning to form while the old vortex 16 is moving farther to the right. The blade 10 nears an upper (per the orientation in FIG. 1C) end of its travel in FIG. 1C, leaving a fully formed vortex 14b in its wake, with vortex 16 still moving outwardly.
In FIG. 1D, blade 10 has reached its full upward excursion and it has stopped moving and is about to reverse with the fully formed vortex 14b still in its wake and the previously formed vortex 16 still moving to the right. The blade 10 then starts downwardly again in FIG. 1E. The vortex 14b is rotating too rapidly to reverse this motion and it is therefore expelled from the blade area by the new airflow around the blade 10. The new airflow 20 is moving up around the tip of the blade 10 towards its wake, while the blade is moving in the direction as shown by arrow 22. Upward flow 20 continues to gain speed as air flows into the vacuum behind (upper per the orientation in FIG. 1F) the blade and the previous vortex 14b is now clear of the blade wake and gaining speed. The blade 10 accelerates towards its center position in FIG. 1G while the air flowing into its wake indicated by arrow 20 is developing a new vortex. In FIG. 1H, with the blade 10 centered and moving downward at maximum velocity as indicated by arrow 22, the air being drawn into the vacuum of the wake has developed into a full vortex 20b. Finally, in FIG. 1I the blade 10 is moved further downward, feeding more air into vortex 20b in its wake. The two previous vortices 14b and 16 are moved toward the right, rotating in opposite directions, one above the center line 18 the other below the center line 18 of blade 10. In this way, a line of oppositely rotating vortices is generated resulting in a highly directional stream of air.
U.S. Pat. No. 4,498,851 indicates that if the vortex shedding effect is disturbed by obstructions in the area, then the air flows from the forward surface of the blade around its trailing edge to the rearward surface of the blade when the motion of the blade reverses. Accordingly, there is only circulation around the trailing edge of the blade and very little outward flow.
In some instances it is, however, it is desirable to provide ducts or channels, i.e. obstructions according to U.S. Pat. No. 4,498,851, to direct the air flow. This may be desirable when certain components are to be cooled by the piezoelectric fan. U.S. Pat. No. 4,498,851 does not provide any teaching for directing air flow generated by a piezoelectric fan where ducts and channels are desired.